(1) Field of the Invention:
The present invention relates to a radial tire for use in passenger cars. More particularly, the invention relates to a passenger car radial tire in which by using steel cords instead of textile cords ordinarily used in a carcass cord layer, durability at tire side portions is increased, and cornering stability, riding comfortability against vibrations, and rolling resistance as motion performances important for the tires are all improved as compared with passenger car radial tires using textile cords.
(2) Prior Art Technique:
Steel cords are customarily used both in a belt cord layer and a carcass cord layer in the case of radial tires for use in trucks and buses. On the other hand, it is a common practice in the case of conventional passenger car radial tires that while the steel cords are used in the belt cord layer, textile cords of such as polyester, nylon, rayon, etc. which have an excellent fatigue resistance are used in the carcass cord layer. However, when these textile cords are used in the carcass cord layer of the passenger car radial tires, there were the following problems:
(1) Resistance to external scratches of the tire side portions formed by rubbing at curbs becomes insufficient. PA0 (2) Since the textile cords generally have low bending and compression rigidity, there is a limit in increasing the rigidity of the tire side portions and obtaining higher motion performances. PA0 (3) since strength becomes insufficient in the case of a relatively large scale passenger car radial tire, a plurality of carcass cord layers need to be used. This results in impeding the motion performance and making the process of tire making more complex.
On the other hand, when steel cords are used in the carcass cord layer of a radial tire for the passenger cars instead of the textile cords, the following problems are expected.
First, since the cords used in the carcass cord layer repeatedly undergo bending and compression deflection for a long period of time during rolling of the tire, material fatigue of the cords proceeds and tenacity of the cords lowers resulting from a fretting phenomenon due to filament yarns in the cords being rubbed with one another. When material fatigue and tenacity reduction become conspicuous, the tire side portions begin to break. For this reason, a technique is needed for improving fatigue resistance and fretting resistance of the steel cords to the extent that they can be used in the carcass cord layer of radial tires for the passenger cars.
Second, when steel cords having a larger bending rigidity than textile cords are use in the carcass cord layer, the rigidity of the tire side portion becomes too high. Thus, there is a problem that while cornering stability is increased, on the other hand, the riding comfortability against vibrations is largely deteriorated. In general, the improvement of cornering stability and the improvement of riding comfortability in the tire are antinomy with each other. In order to obtain the tires excellent in both of the motion performances, a tire technique is required to overcome this contradictory problem.
Under this background, it has been considered generally difficult to use the steel cords in the carcass cord layer of the passenger car radial tire due to the insufficient fatigue resistance and deterioration in a part of the motion performance, particularly, such as riding comfortability against vibrations.
As an example of conventional passenger car radial tires in which the steel cords are used in the carcass cord layer, is a pneumatic radial tire disclosed in Japanese patent application Laid-open No. 58-221,703. This tire is characterized by using metal cords of a 3.times.3 construction in which three strands each having three metal filaments twisted are twisted together and the diameter of the filaments is from 0.08 to 0.15 mm. This publication describes as its effects that side cut burst resistance, cornering stability and high speed durability can be attained.
However, it has been made clear by study and examination of the tires disclosed in the above laid-open publication that the following problems are included therein.
First, the 3.times.3 construction in the steel cord as one of the constituent requisites in this tire is a socalled strand construction. When the steel cords of this construction are used in a passenger car radial tire having a large deflection of the carcass cord layer and a large movement of the cords, not in a truck and bus radial tire for use under a high internal pressure, tenacity of the cords are largely decreased due to a fretting wear at point contacts among filament yarns and particularly among the strands during running. This renders the application of such steel cords to the passenger car radial tire infavorable. With respect to fatigue resistance, high stress is repeatedly and locally applied onto the surfaces of the filaments in the strand construction including the point contacts. Therefore, even when small diameter filaments having higher flexibility are used, there is a great fear that fatigue rupture occurs. Further, when the diameter of the filament yarn is in a range of 0.08 to 0.15 mm which is one of the constituent requirements, it is difficult to make the bending and compression rigidity of the cords themselves high, so that expected motion performance, can hardly be obtained.